ocaml: phase 2 for/while loops (+5 tests, 194 total)

Parser: for i = lo to|downto hi do body done, while cond do body done.
AST: (:for NAME LO HI :ascend|:descend BODY) and (:while COND BODY).
Eval re-binds the loop var per iteration; both forms evaluate to unit.

plans/ocaml-on-sx.md

@@ -155,7 +155,8 @@ SX CEK evaluator (both JS and OCaml hosts)
 - [x] Unit `()` value; `ignore`.
 - [x] References: `ref`, `!`, `:=`.
 - [ ] Mutable record fields.
-- [ ] `for i = lo to hi do ... done` loop; `while cond do ... done`.
+- [x] `for i = lo to hi do ... done` loop; `while cond do ... done` (incl.
+      `downto` direction).
 - [ ] `try`/`with` — maps to SX `guard`; `raise` via perform.
 - [ ] Tests in `lib/ocaml/tests/eval.sx` — 50+ tests, pure + imperative.
 
@@ -320,6 +321,9 @@ the "mother tongue" closure: OCaml → SX → OCaml. This means:
 
 _Newest first._
 
+- 2026-05-08 Phase 2 — `for`/`while` loops. `(:for NAME LO HI DIR BODY)`
+  with `:ascend`/`:descend` direction (`to`/`downto`); `(:while COND BODY)`.
+  Both eval to unit and re-bind the loop var per iteration. 194/194 (+5).
 - 2026-05-08 Phase 2 — references (`ref`/`!`/`:=`). `ref` is a builtin
   that boxes its argument in a one-element list (the mutable cell);
   prefix `!` parses to `(:deref EXPR)` and reads `(nth cell 0)`; `:=`